1. Field of the Invention
The invention relates to pulverulent organometallic compounds intended for forming a film of tin oxide on a transparent substrate such as of glass, by a pyrolysis technique. The invention also concerns a method of depositing by pyrolysis and the substrate thus coated.
2. Discussion of the Background.
It is known to coat substrates of glass with thin films of a doped metallic oxide, and notably films of tin oxide doped with a halogen-containing, generally fluorine-containing, compound in order to give them certain properties, including properties affecting infrared radiation and/or electrical conduction properties. In this way it is possible to obtain low-emissivity or heating glass panes.
Various techniques exist for depositing films onto a substrate, including the technique known as powder pyrolysis. This technique involves projecting suitable organometallic compounds, i.e., halogenated stannic compounds, in the form of particles in suspension in a carrier as, directly onto the surface of a glass substrate raised to a high temperature, for example on the order of 400.degree. to 650.degree. C. On contact with the hot glass, the particles decompose leaving an oxide film. This is a proven technique, which has the advantage of allowing a continuous deposition onto a ribbon glass in a production line of a "float" installation and which enables quality coatings to be obtained. Suitable devices for carrying out this technique are described for example, in EP-B-0 125 153 (U.S. Pat. No. 4,533,571) and EP-A-0 374 023 (U.S. Pat. No. 5,005,769).
Various types of pulverulent organometallic compounds are suitable for obtaining films of tin oxide doped with fluorine (SnO.sub.2 :F). Thus, there are known compounds simultaneously containing tin and fluorine, such as dipropyl tin ditrifluoroacetate (C.sub.3 H.sub.7).sub.2 Sn(CF.sub.3 COO).sub.2 and dibutyl tin ditrifluoroacetate (C.sub.4 H.sub.9).sub.2 Sn(CF.sub.3 COO).sub.2 described in EP-B-0 106 744 and dibutyl tin difluoride (C.sub.4 H.sub.9).sub.2 SnF.sub.2, termed DBTF, a synthesis of which is described in EP-B-0 178 956.
It is also known from EP-B-0 039 256 to use a stannic compound without halogen, such an dibutyl tin oxide (n-C.sub.4 H.sub.9).sub.2 SnO, termed DBTO, which is associated with another compound carrying halogen, such as DBTF. For all these compounds, particles having fairly small particle sizes are chosen, with at most mean particle diameters on the order of 15 to 20 microns and preferably particle diameters all below 20 microns, in order to facilitate bringing them into suspension in the carrier gas and their homogeneous flow from the distribution devices to the surface of the substrate to be coated.
As noted above, the coatings obtained from these pulverulent compounds have an optical appearance and satisfactory performance. Thus, in conformity with the teaching of EP-B-0 125 153 and EP-A-0 374 023, already referred to above, it is possible to obtain films of SnO.sub.2 :F having a uniform thickness and an emissivity which can be up to 0.25 from DBTF powders having a granulometry of less than 20 microns.
Attempts have been made to reduce still further the emissivity of such pyrolyzed films by altering the deposition conditions, such as the temperature of the substrate, or by increasing the thickness of the films. There are limits, however, to these optimizations if only because they create additional costs in terms of energy consumed and/or raw materials. Moreover, increasing the thickness of the film may modify its optical appearance in an undesired manner and the gain in emissivity progressively dwindles with this increase in thickness.
A need continues to exist for methods of improving, by various means, the properties of conductivity and/or electrical conductivity of the pyrolyzed films of doped tin oxide, and to achieve this result without causing difficulties and/or additional costs in production.